TASI systems are well-known and consist basically in increasing the number of signal sources that can be switched over a fixed number of transmission lines by connecting a talker and a listener only when the talker is actually speaking. One example of such a system is described in U.S. Pat. No. 3,030,447 issued Apr. 17, l962 to Saal.
Most conventional detectors operate on the analog (non-digital) vocal signal and consist in computing the mean power value of the signal and in comparing this value with a pre-determined decision threshold. More recent systems consist in periodically sampling the amplitude of voice-frequency signals and in translating these amplitude values into digital form (see, for example, U.S. Pat. No. 3,712,959 granted Jan. 23, 1973 to Fariello and U.S. Pat. No. 3,832,491 granted Aug. 27, 1974 to Sciulli). However, the decision reached concerning the status of a voice channel is based only on the amplitude of the vocal signal and a distinction is made only between noise and silence.
In present detectors, there is a certain delay before the beginning of the identification of speech so as to prevent undesired pulse noises which could cause the unwanted activation of a transmission channel. This delay is required in order to ensure that the talker has really began to speak and is an inverse function of the signal amplitude. This solution, while avoiding false activation, reduces the intelligibility of the message since there is a chopping of the consonants of low amplitude which, however, contain very useful information. Indeed, the differences between the sounds "ta" and "da" or "pa" and "ba" are condensed in the first milliseconds. Furthermore, in presently known detectors, since consonants include a lot of information and since they are of low amplitude, there is a tendency to consider as speech all signals having a relatively low amplitude. This results in considering as speech: white noises of various origins which are inherent to all transmission channels; and echoes, i.e., vowels of high amplitude which the other talker transmits and which, by interference, are present in the channel under consideration. These echoes are evidently reduced but have sufficient amplitude to cause a reactivation.